don't confuse \transposition and \transpose. \transposition sets the
transposition of the instrument playing. This is used for getting cue
note transpositions and MIDI output correct. \transpose changes the
pitches of a music expression. The above was interpreted as

Thanks. So I can't do what I want :-(

I know someone said there was a macro in jEdit or emacs or something
that would transpose parts, but what I was hoping I could do was enter
the part straight from treble clef, and tell lilypond that it was
written in Bb. Looking at the example for the horn certainly makes it
look like that is possible ...

I've been entering pieces in concert C and they are are printed in the
correct instrument key with this in the code -